The present invention relates to a heavy duty radial tire accommodated to a 15 degree taper center drop rim. More particularly, the present invention is directed to an improved bead structure capable of preventing a carcass ply separation failure and improving bead durability.
In general, heavy duty radial tires such as truck/bus tires used under high-pressure and heavy-load conditions are provided with a steel cord carcass ply which is turned up around bead cores, and the tires are used together with a 15 degree taper center drop rim (for example, R06-1 in JATMA) wherein the rim base (bead seat) is tapered at 15 degrees to obtain a large engaging force between the tire and rim. The heavy duty tires are often used under severe condition such that the tire deflection reaches up to 15% of the tire sectional height. Under such severe conditions, a stress is liable to concentrate on the cord ends of the carcass turnup portion, and thus a separation failure from the surrounding rubber is liable to occur.
In such a heavy duty radial tire, the angle of the carcass cords is substantially 90 degrees to the tire equator, which is regarded as a cause of the concentration of the deformation because it is difficult for the carcass to lean circumferentially of the tire and also toward the rim flange.
Hitherto, therefore, as shown in FIGS. 6 and 7, the bead portions of a heavy duty radial tire have been provided with a steel cord bead reinforcing layer (d) extending along the carcass turnup portion (c1) to increase the rigidity and thus to decrease deformation.
The cords of the bead reinforcing layer (d) are inclined circumferentially of the tire for example at an angle of not more than 60 degrees to the circumferential direction of the tire so as to lean easily not only toward the rim flange (j1) but also circumferentially of the tire.
In FIG. 6, the bead filler (d) is lower in height than the outer end (ct) of the turnup portion (c1) of the carcass (c), and in order to reduce the deformation of the tire during rolling which is transmitted to the tread, belt, carcass main portion (c2), bead apex (e) and carcass turnup portion (c1),
the bead filler (bead reinforcing layer) is provided to support the outside of the carcass turnup portion (c1) against the deformation to prevent the separation between the cord ends of the turnup portion (c1) and the rubber.
In FIG. 7, the bead reinforcing layer (d) is higher in height than the outer end (ct) of the carcass turnup portion (c1). In this case, the deformation of the tire during rolling is transmitted to the tread, belt, carcass main portion (c2), bead apex (e) and then to the bead reinforcing layer (d) before the carcass turnup portion (c1). Thus, the bead reinforcing layer further reduces the deformation around the outer end (ct).
As explained above, it is known to be effective to reinforce the bead portion of a heavy duty pneumatic tire by disposing a bead reinforcing layer in the bead portion.
However, the present inventors have discovered that it is also possible to improve the durability of the bead portion by decreasing the difference B-D of the inside diameter (B) of the bead core (f) of the tire from the rim diameter (D) of a 15 degree taper center drop rim. Here, the inside diameter (B) is of the tire not mounted on the rim as shown in FIG. 6.
In a conventional heavy duty tire with a steel cord carcass (c) reinforced by a bead filler (reinforcing layer) (d) extending from the radially inside of a bead core (f) toward the radially outside, as the rigidity of the bead portion is very high, the difference B-D between the inside diameter (B) of the bead core and the rim diameter (D) is set at a relatively large value of for example not less than 2 mm, giving priority to the rim mounting.
However, if the diameter difference B-D is too large, the tire bead base (g) has a tendency to move axially outwardly along the rim seat (h) when the tire is inflated, and the so called rotational stress of the carcass ply around the bead core (f) increases and the deformation of the bead portion also increases. Further, the bead compression between the bead core (f) and the rim seat (h) decreases, which produces the problems of carcass ply slipping-out, carcass ply turnup loose and the like.
On the other hand, the rubber thickness between the carcass cords and the bead filler cords, measured at the bead filler end (dt) in FIG. 6 and the carcass turnup end (ct) in FIG. 7, is usually set at a very small value of under 1.0 mm. Thus, in spite of the provision of the bead filler (d), a stress concentration occurs on the ends (dt) in FIG. 6 and (ct) in FIG. 7, and as a result, a separation failure (PTL: Ply Turnup Loose) starting from these ends is liable to occur.